1. Field of the Invention
The present invention relates to a maintenance technique for ink jet printers having an ink circulation mechanism.
2. Description of the Background Art
In the case of an ink jet printer which prints images by ejecting ink from nozzles, when the print process has not been performed over a long time, the solvent of ink is evaporated or volatilized in the vicinity of ink jet heads so that the viscosity of ink is increased to degrade the print performance of the ink jet printer. Because of this, as the need arises, a maintenance operation is performed, for example, by suctioning the ink, cleaning nozzles and so forth.
Japanese Patent Published Application No. 2003-089226 describes a technique to perform maintenance by measuring the time elapsed after the previous maintenance and selecting an appropriate maintenance operation in accordance with the elapsed time. Also, Japanese Patent Published Application No. 2006-205744 describes a technique to perform maintenance by driving a head actuator, detecting entry of air bubbles, increase of the viscosity of ink and attaching of paper dust on the basis of the residual vibration, and selecting an appropriate maintenance operation in accordance with the results of the detection.
On the other hand, in recent years, it is proposed to provide an ink circulation route in the body of an ink jet printer to enable ink circulation for the purpose of improving the reliability of the print process as described in Japanese Patent Published Application No. Hei 11-342634. In the case of the ink jet printer having such an ink circulation mechanism, even if a nozzle clogs up with bubbles or debris, quick recovery is possible, and the ink circulation through the ink chamber of an ink jet head serves to sweep away high viscosity ink to the ink circulation route.
However, even if ink is circulated, when the ink jet printer is unused for a long time, the viscosity of ink is further increased so that maintenance such as suctioning ink becomes required in the same manner as in the conventional ink jet printers. It is preferred to perform the conventional maintenance only at a minimum frequency from the view point of saving ink because ink is consumed in vain by the process of suctioning ink and so forth during maintenance. Particularly, in the case of the ink jet printer having such an ink circulation mechanism, the thickened ink can be recovered by ink circulation as long as the viscosity of ink is not excessively increased, and thereby it is important to find out when it becomes necessary for ink to perform maintenance which consumes a certain amount of ink.
Because of this, an appropriate maintenance operation may be selected in accordance with the elapsed time after the previous maintenance as described in Japanese Patent Published Application No. 2003-089226. However, the elapsed time is not necessarily reflected in the viscosity of ink in a proportional manner, and thereby ink may unnecessarily be consumed, or necessary maintenance may not have been performed so that the viscosity of ink is increased to be too high to recover the thickened ink. Alternatively, it can be considered to perform maintenance by selecting an appropriate maintenance operation on the basis of the residual vibration as described in Japanese Patent Published Application No. 2006-205744. However, in this case, there is a problem that the production cost rises because of the circuit provided for detecting the residual vibration to increase the scale of the printer structure and the production cost.